The present invention relates to an apparatus for deploying an object to an underwater target position, the apparatus being provided with a beacon to transmit acoustic rays and a plurality of thrusters to control positioning of the apparatus with respect to the underwater target position.
Such an apparatus is known from WO 99/61307.
The prior art apparatus is used for deploying and/or recovering loads up to 1000 tons or more on the seabed at great depths, for instance, up to 3,000 meter or more. During deployment, the apparatus is controlled by controlling equipment on board of a vessel floating on the sea surface. The controlling equipment needs to know the exact location of the apparatus as accurate as possible. To that end, the beacon on board of the apparatus transmits acoustic rays through the sea water to the vessel. An appropriate acoustic receiver receives these acoustic rays and converts them into electrical signals used to calculate the position of the apparatus with respect to the vessel.
However, it is found that with increasing depth of the apparatus below the sea water the accuracy of the location measurement decreases due to bending of the acoustic rays in the sea water.
The object of the invention is therefore to further enhance the accuracy of the location measurement of such an apparatus during use in sea water or any other fluid. Moreover, such location measurement is needed on-line (real-time).
To obtain this object, the apparatus as defined at the outset is characterized in that it is provided with a sound velocity meter to measure velocity of sound in a fluid surrounding said apparatus. Thus, the velocity of sound at a certain location in the fluid can be continuously measured and used to update a sound velocity profile, i.e., data as to the sound velocity as a function of depth in the fluid. From these data, local bending of the acoustic rays can be determined on-line (real-time). So far, such on-line determination has not been possible. This allows corrections of location measurements in real-time.
In a preferred embodiment, the thrusters comprise a first set of thrusters arranged to provide a torque control function and a second set of thrusters arranged to provide at least a translation function, each thruster of the second set of thirsters being provided with a rotary actuator.
This is a very advantageous embodiment. Only two thrusters are necessary to prevent any undesired rotation of the apparatus attached to the load during deployment thus avoiding all problems related to twisting and turning of hoist wire the load, as already explained in WO 99/61307. Moreover, only two rotatable truss are needed to control positioning of the apparatus with its load attached to it to the desired horizontal coordinates. Thus, prior to lowering the load with the apparatus the apparatus can move the load to the desired horizontal coordinates and when these coordinates have been reached the hoist wire(s) can lower the load to its desired location on the seabed while the thrusters keep the load on the desired coordinates and prevent any undesired rotation of the load. Only when the desired target position on the seabed is reached a possible rotation of the load to a desired orientation need be carried out by the thrusters dedicated to the torque control.
It is observed that rotatable thrusters on an underwater apparatus for deploying loads to a desired position are known from U.S. Pat. No. 5,898,746.
The apparatus is preferably provided with load cells to measure weight of the load attached to the apparatus. When the load is put on the seabed by this weight suddenly decreases. Thus, a signal indicating that the weight of the load suddenly decreases can be used to determine when the apparatus may be detached from the load.
The invention also relates to a processing arrangement arranged to drive an apparatus for deploying an object to an underwater target position, the apparatus being provided with a beacon to transmit acoustic rays, a plurality of thrusters to control positioning of the apparatus with respect to the underwater target position, and a sound velocity meter to measure velocity of sound in a fluid surrounding the apparatus, the processing arrangement being provided with an acoustic receiver to receive the acoustic rays, the processing arrangement is arranged to use data derived from the acoustic rays in a calculation to determine the position of the apparatus characterized in that the processing arrangement is armed to receive online sound velocity meter data from the sound velocity meter to determine a sound velocity profile in the fluid and to calculate from the sound velocity profile bending of the acoustic rays transmitted by the apparatus through the fluid and to use this in the calculation to determine the position of the apparatus in real-time.
Such a processing arrangement is able to control driving of said apparatus to a desired location in a desired orientation with very high accuracy, even at great depth under water. While the apparatus with its load is lowered, the processing arrangement constantly receives sound velocity data and determines a sound velocity profile comprising sound velocity data from the water surface to the depth of the apparatus. The processing arrangement uses these data to determine acoustic ray bending as a function of the depth in the water and thus to correct any position calculation of the apparatus.
Such a processing arrangement may be on board of a vessel floating on the water surface. However, it is to be understood that part of the functionality of determining the sound velocity profile and calculating the acoustic ray bending may be carried out by one or more processors located elsewhere, even on board of the apparatus itself.
Preferably, a further sound velocity meter is provided just below the water surface to provide actual data regarding any ray bending in the water surface layers and thus to further correct any position calculation of the apparatus.
Reception of the acoustic rays transmitted by the apparatus is preferably performed by an acoustic array attached to the hull of the vessel.
In a very preferred embodiment, the vessel, the acoustic array and the apparatus are all provided with a distinct gyrocompass measuring respective heaves, rolls and pitches. Output data from these gyrocompass are used to further increase accuracy of the position measurement of the apparatus.
The invention also relates to a system comprising such a vessel and an apparatus together.
The invention also relates to a method of driving an apparatus for deploying an object to an underwater target position, the apparatus being provided with a beacon to transmit acoustic rays, a plurality of thrusters to control positioning of the apparatus with respect to the underwater target position, and a sound velocity meter to measure velocity of sound in a fluid surrounding the apparatus, the method comprising the steps of:
receiving the acoustic rays,
using data derived from the acoustic rays in a calculation to determine the position of the apparatus
characterized by the steps of:
receiving sound velocity meter data from the sound velocity meter and determining a sound velocity profile in the fluid, and
calculating from the sound velocity profile bending of the acoustic rays transmitted by the apparatus through the fluid and to use this in the calculation to determine the position of the apparatus.
This method may be entirely controlled by a suitable computer program after being loaded by the processing arrangement Therefore, the invention also relates to a computer program product comprising data and instructions that after being loaded by a processing arrangement provides said arrangement with the capacity to carry out a method as defined above.
Also a data carrier provided with such a computer program product is claimed.